This invention relates generally to optical fiber, and more particularly relates to active, doped optical fiber.
The development of doped, actively-pumped optical fiber has been fundamental for enabling the application of optical fiber systems, such as fiber-based laser systems, to the fields of telecommunications, sensor systems, and electro-optic systems, as well as for industrial applications such as high-speed material cutting, welding, and micromachining, among other applications. In one example, a doped, actively-pumped optical fiber generally consists of a fiber core region including a core material that is doped with a selected dopant for causing lasing of the core material at a selected wavelength. The fiber core region is surrounded by a first cladding region though which pump light is introduced to the fiber, e.g., from one or both ends of the fiber. The pump light in the first cladding region is absorbed by the core region along the fiber length, causing population inversion and lasing of the material in the core region, with the core region carrying the laser light through the fiber. The first cladding region is typically surrounded by a second cladding region that is provided as an outer protective layer for the fiber and to confine the pump light to the first cladding and core regions.
In operation, the core region of a doped, actively-pumped optical fiber generates heat as the input pump signal is converted to a laser signal, and this heating primarily occurs in the core material, due to, e.g., quantum defects of the lasing process and the degree of pump and signal absorption. Because the core region generally has a very large surface-to-volume ratio in a conventional fiber geometry, the heat generated in the core region dissipates to the outer cladding regions of the fiber. This heat generation and dissipation can result in thermal damage of the fiber core material as well as the materials of both the first fiber cladding and second fiber cladding. Heat generation and dissipation can also cause detrimental thermo-optical effects during fiber laser operation, such as multi-mode instability, also referred to as transverse mode instability and modal instability, of the laser signal in the core of the fiber.
Conventionally, a lasing pump signal is introduced into a doped, actively-pumped fiber system through the first cladding layer of the fiber, from one or both ends of the fiber, with the pump light crossing into the fiber core as the pump light progresses down the length of the fiber. In the doped fiber core, the pump energy is absorbed and converted to the target laser wavelength. The geometry and material composition of the fiber core region and the fiber cladding regions are generally homogeneous along substantially the entire length of the fiber, resulting in nonuniform, with respect to length, distribution of pump energy and correspondingly non uniform heat dissipation along the fiber length. The magnitude of this absorption and heating depends on the doping and other characteristics of the fiber materials as well as the choice of pump and target laser wavelengths. The heat that is generated in the doped fiber core occurs at regions of highest signal power and highest gain, i.e., the site of highest conversion of pump to signal power. This is in general relatively near to locations along the length of the doped fiber where pump power is introduced into the first cladding region of the fiber. Thus, there can be regions of fiber along which heat generation is pronounced, and there can exist specific sites along the fiber length at which heat generation is extreme.
Many fiber laser applications, e.g., multi-kW fiber lasers and multi-W amplifiers for industrial material processing and telecommunications, require robust and sustained fiber operation over long durations. But as a result of heating during fiber laser operation, the requirements of many such applications are not attainable with conventional fiber systems. As a result, the design and system limitations imposed by the need to prohibit thermal damage of fiber lasers prohibits the ability to achieve the reliable optical power scaling required by many important fiber applications.